1. Field of the Invention
The present invention relates to electric spark building-up of metal on wearing parts and more particularly to methods of restoring rolls and to devices for carrying out these methods.
Most successfully the invention can be used for hot and cold rolling.
2. Description of the Prior Art
It is known from operating rolling equipment that in the course of rolling, the working surface of rolls is subjected to continuous wear due to friction. Because of irregular wear the roll working surface, i.e. its cross dimensions change, thereby altering specified shape and dimensions of the rolled product. As the wear allowance approaches its permissible limit, it becomes necessary to restore the working surface of rolls to its initial shape in order to avoid an excessive waste of metal being rolled and to prevent spoilage of the finished product.
The simplest method for rebuilding worn rolls consists in removing the surface layers thereof so that their working profile acquire their initial shape. However, as a result of such treatment which is repeated many times during the service life of rolls, the diameter thereof decreases to a size which necessitates their replacement. Moreover, such treatment is a rather labour-consuming one.
In addition, the above method of treatment necessitates dismantling the worn rolls and mounting the restored ones, which makes it necessary to stop the whole production process. This, in turn, results in lower productivity of the rolling mill and entailes the increase of operating costs.
More economical are methods of rebuilding rolls by which a metal layer is built up on the worn surface of rolls, with the thickness of the built-up layer exceeding the depth of wear. Excess metal is then removed from the surface of the built-up layer by machining.
There are known various methods of rebuilding rolls by metal surfacing. However, these methods are mainly applicable for rebuilding rolls having a simple configuration, and are not suitable for restoring rolls made of cast-iron. In addition, the use of such methods necessitates machining the roll surface and hence, their dismantling and mounting.
Known in the art is also the method for strengthening the working surface of rolls by metal surfacing using an electric spark treatment. This method can be used, in particular, for restoring rolls by means of build-up of metal on their working surface.
This method of electric spark treatment of rolls (Ivanov G.P. Technologiya electroiskrovogo uprochneniya instrumentov i detalei machin, M., Mashinostroenie, 1957, p. 177, FIG. 107) consists in that a voltage from a unipolar current source is applied to a workpiece (cathode) to be strengthened and to an electrode (anode) interacting with the working surface of the workpiece.
In the process of electric spark discharge produced in a conventional manner the material of the electrode is transferred onto the roll surface to be strengthened. To provide a high operating durability of the roll being treated by this method it is necessary that the thickness of the built-up layer be as great as possible. However, the thickness of the layer thus formed is limited by the limit layer phenomenon consisting in that there exists a relationship between the thickness of the built-up layer and the welding current, i.e. there exists a maximum thickness of the built-up layer for a given current and voltage. It means that irrespective of the duration of treatment it is impossible to form a layer thicker than the limit layer the thickness of which is determined in each particular case by the current and voltage. According to the experimentally obtained data the maximum thickness of the built-up layer ranges depending on the current applied from 0.005 to 2.0 mm.
Thus, as soon as the voltage from the unipolar current source is applied, the metal begins to build up on the surface being treated as a result of the transfer of the electrode material. After the thickness of the layer assumes a value which is maximum for the current applied (U=const, I=const), further building-up of metal does not occur in spite of the fact that the electric sparking continues. In this case a further increase of the thickness of the built-up layer can be achieved only by increasing the current and voltage.
However, increasing the thickness of the metal layer by increasing the current and voltage affects the smoothness of the surface being treated, which necessitates machining of the built-up layer to impart the desired smoothness to the working surface of the roll. These disadvantages of the above method of rebuilding rolls are responsible for its limited application. In addition, this method as well as the other known methods require dismantling of rolls for their repair and mounting them again after their being repaired.
An object of the present invention is to provide a method for electric spark building-up of metal on rolls, permitting the initial shape of their working surface to be maintained in the course of operation.
Another object of the present invention is the provision of a device for carrying out the above method permitting the initial shape of the working surface of rolls to be maintained in the course of operation.
Still another object of the present invention is to provide a method for maintaining the initial shape of the working surface of rolls and a device for carrying out this method, which provide for a minimum waste of the metal being rolled.
Further object of the invention is to reduce repair costs of rolls.